WASHINGTON (Reuters) - A bipartisan group in the U.S. House of Representatives is attempting to craft a bill that would give millions of illegal immigrants a way to become citizens, House aides said on Thursday, mirroring an effort in the Senate.

One of the aides said the House legislation would be tougher in some ways than the plan put forward on Monday by four Democrats and four Republicans in the U.S. Senate.

The Senate proposal, which has not yet been put into legislative form, would require illegal immigrants to undergo background checks and pay back taxes and penalties before obtaining temporary legal status in the United States.

The House aide, who requested anonymity, said the House proposal was "tougher in terms of the application process," but would not go into detail.

The House group includes Republicans Mario Diaz-Balart of Florida, John Carter of Texas and Raul Labrador of Idaho, and Democrats Luis Gutierrez of Illinois and Zoe Lofgren from California. The latter is the top Democrat on a House Judiciary subcommittee overseeing immigration.

Another congressional aide said the House legislation was 90 percent complete and included a similar provision to the Senate plan that would make it harder for employers to knowingly hire illegal immigrants.

"We are in touch with our counterparts in the House," New York Democratic Senator Charles Schumer, one of the "Gang of 8" senators who released the Senate proposal on Monday, told a news conference.

"We believe that they're moving along on a set of principles that will be fairly similar to ours, not completely the same."

Any major changes to the immigration law must win support in the Republican-controlled House, where conservatives have in the past rejected what they consider would be an amnesty for those who entered the country illegally.

The fact that the bipartisan group of House lawmakers is likely to include a "path to citizenship" in its proposal is no guarantee that the idea will overcome expected opposition from conservatives, but it could help because it shows some House Republicans are on board.

However, it was unclear on Thursday whether Labrador, one of the House group's newest members, would sign off on the path to citizenship.

"I don't think there should be a new path to citizenship for the adults," Labrador told Reuters. "I Believe that in the House it will be very difficult to pass any bill that has a pathway to citizenship," he said.

Labrador has proposed a program that would allow illegal immigrants who have jobs to apply for temporary but renewable work visas.

The House group, with a membership that has varied, has been meeting privately for about four years. Lawmakers were ready to unveil their immigration legislation in 2012, but shelved the bill because they knew it would not go anywhere in an election year.